It is known to produce hydrocarbons by bringing a gas mixture composed of hydrogen and carbon monoxide into contact with a catalyst at an elevated temperature under pressure, as the synthesis of hydrocarbons by the so-called Fischer-Tropsch (FT) process.
The catalytic hydrogenation of carbon monoxide yields a mixture composed of paraffins and olefins having from 1 to 40 carbon atoms, depending upon the catalyst used, and the reaction conditions applied, and it sometimes yields compounds containing oxygen such as alcohol, aldehyde, ketone, ester, or aliphatic acid. Further, under specific synthetic conditions, a small amount of aromatic hydrocarbons is formed.
Remarkable activity for hydrogenation of carbon monoxide is shown by Periodic Group VIII elements, particularly, iron, cobalt, nickel and ruthenium. Even though these elements are used, distributions of the products and compositions thereof vary remarkably, depending upon the kind and the amounts of the elements. It is already known that nickel, cobalt, and ruthenium type catalysts produce mainly a mixture of unbranched saturated hydrocarbons, and that hydrocarbons containing unsaturated aliphatic compounds and oxygen-containing compounds, particularly aliphatic primary alcohols, can be produced using an iron-containing catalyst.
However, the iron type catalysts produce a mixed product rich in unsaturated hydrocarbons, but amounts of carbon dioxide gas and oxygen-containing compounds formed as by-products are large, and selectivity of hydrocarbons is inferior. On the other hand, the ruthenium type catalysts have a high FT reaction activity which is several times higher than that of the iron type catalysts, but they produce a product having a larger amount of saturated hydrocarbons, and have high dependence on the reaction pressure and reaction temperature, and there is a tendency that the chain growth probability: .alpha. value of Schultz-Flory law, which is often used as an index of FT reaction, greatly decreases with increases of the reaction temperature. For such a reason, it has been said that it is very difficult to selectively produce a reaction product containing a large amount of olefinic hydrocarbons using the ruthenium type catalyst. Further, since the iron type catalysts used in the known processes have low reaction activity, iron content per unit catalyst is large, and the reaction is carried out at a high temperature. Carbon monoxide forms carbon at a high reaction temperature according to Boudouard's equilibrium. Deposition of the carbon causes deactivation of the catalyst and destruction of the catalyst structure; consequently, the catalyst life is remarkably reduced. Further, since the iron content is high, there is a problem in that a sintering phenomenon occurs to a significant extent, and the useful life of the catalyst is unsatisfactory.
Catalysts for hydrogenating carbon monoxide ordinary used are sensitive to poisoning, most particularly with respect to sulfur compounds. Therefore, it is necessary to purify the raw materials of carbon monoxide and hydrogen, and thus it is urgently required to develop a sulfur resisting catalyst considering economy of the process. Further, in the case of producing olefins by hydrogenating carbon monoxide, though a higher conversion can be attained with increase of hydrogen partial pressure, hydrogenation of the olefins formed is accelerated at the same time, to result in deterioration of olefin selectivity for the product. Moreover, it is more difficult to increase the production ratio of ethylene/ethane than to increase the production ratio of propylene/propane, in the viewpoint of equilibrium in the FT reaction.
In recent years, a conversion process for selectively obtaining liquid hydrocarbons, and particularly a gasoline fraction from a synthesis gas, which comprises using zeolite and a carbon monoxide reduction catalyst (FT synthetic catalyst and methanol synthetic catalyst) in combination has been studied.
This conversion process includes a two-stage conversion process wherein the reactions are carried out in different reactors, and a one-stage process wherein a catalyst comprising a metal component active to reduction of carbon monoxide supported on specified zeolite is used or a mixed catalyst which is a physical mixture of a carbon monoxide reduction catalyst and a specified zeolite is used.
The one-stage process is generally expected to be a more economic process than the two-stage process, because the process is simplified. However, satisfactory results in reaction activity or distribution or composition of the formed hydrocarbon product are not obtained in the one-stage process, because the optimum conditions (particularly, reaction temperature and pressure) for the two catalysts are different from each other, as compared with the two-stage process wherein the carbon monoxide reduction catalyst and the zeolite catalyst can be used under the optimum conditions, respectively. For example, processes for selectively producing a gasoline fraction in one stage using a ruthenium-containing catalyst of this kind are known in U.S. Pat. No. 4,157,338 and Japanese Patent Application (OPI) Nos. 19386/83, 127784/83, and 192834/83 (the term "OPI" as used herein refers to a "published unexamined Japanese Patent Application open to public inspection"). However, these processes have disadvantages in that the amount of methane formed is large and yield of the gasoline fraction is low, conversion of carbon monoxide is low and/or high reaction pressure is required.
As is described above, it is very difficult to selectively produce hydrocarbons of the specified useful component or the specified gasoline fraction by the prior processes, because undesirable methane or carbon dioxide gas is formed in a relatively large amount as the by-product, selectivity for the desired hydrocarbons is low, and carbon atom distribution of the formed hydrocarbons is very broad and hydrocarbons in a range of gas to wax are formed.